Before petroleum plastics, three natural-polymer materials did the job: casein plastic from milk protein, shellac from insect resin, and cellulose-based materials from plant fibre. Casein plastic, sold as Galalith, was invented in 1897 and made in tens of thousands of tons a year before petroleum plastics replaced it; it is hard and workable but takes up water until it is hardened. Choose by the property needed, and leave the explosive cellulose chemistry to the plastics articles.
Part of Organic Chemistry
Moldable and coating materials made from casein, shellac and cellulose when petrochemical plastics are unavailable.
Three Materials, Three Behaviours
Casein plastic. Milk protein kneaded into a dough, moulded, dried, hardened and polished. The corpus covers it in Casein Plastic (the page gives the 1897 date for Galalith and the hardening step) and Casein Extraction. The hardening step uses formaldehyde, a toxic reagent that has its own page, Formaldehyde Hardening; this article does not repeat it.
Shellac. Insect resin that dissolves in alcohol, then dries to a hard film and melts when warmed, so it is both a varnish and a thermoplastic. See Shellac.
Cellulose. The carrier lists cellulose as a plastic-alternative source. Alternative Polymers describes the benign cellulose products such as treated parchment. The early thermoplastic made from nitrated cellulose, Celluloid, and the first synthetic thermoset, Bakelite, are written as chemistry and history in their own articles. This article gives no nitration of cellulose, because the margin between a plastic-grade and an explosive-grade product is narrow and cannot be controlled without instruments.
What You Need
- Skim milk and vinegar for casein; the casein pages give the method.
- Seedlac and alcohol for shellac.
- Moulds, lathe or hand tools, and polishing materials.
- A hardening bath with its own safe-handling arrangements for casein.
- Drying space and time. Casein pieces dry slowly to avoid cracking.
Method: Choosing A Material
Step 1: Decide whether the part is rigid, a film or a coating.
Step 2: Casein for small rigid parts (buttons, handles); shellac for coatings, insulation films and varnish; parchment-type cellulose for sheets.
Step 3: Test water exposure on a sample before committing, because unhardened casein swells.
Step 4: Record the preparation.
Worked Example: Comparing Two Insulating Films
The shellac page gives a dielectric strength of roughly 30 kV per millimetre, and melting at 75-80 °C. A film 0.1 mm thick would then hold about 0.1 × 30 = 3 kV, and softens in hot water. These figures come from that page and the film thickness is an illustration. The calculation shows the trade: good electrical insulation, but a thermoplastic that melts below boiling-water temperature, which makes shellac wrong for anything that runs hot.
Common Mistakes
- Expecting an unhardened casein piece to survive water.
- Using shellac where the part gets hot.
- Attempting nitrocellulose for a plastic. It is an explosive hazard.
- Skipping ventilation at the hardening step.
Safety
Formaldehyde, alcohol vapour and cellulose nitrates
Formaldehyde hardening solution is toxic and an irritant; use it only as the formaldehyde page directs, in the open. Alcohol solvents burn. Do not nitrate cellulose: nitrocellulose of high nitrogen content is an explosive and no home method can hold it safely below that grade. The plastics articles (Plastics from Scratch, Plastics Manufacturing) set out the history without a recipe.
What’s Next
- Learn next: Plastics from Scratch — the synthetic route
- Combine with: Polymerization — what makes a plastic a plastic